The coronal mass ejection (CME) produced by the X4.5 solar flare on September 14, 2024, impacted the Earth’s magnetic field at 23:49 UTC on September 16, triggering a G4 – Severe geomagnetic storm.
The geomagnetic K-index of 5 (G1 – Minor geomagnetic storm) threshold was reached at 01:05 UTC on September 17, followed by a K-index of 6 (G2 – Moderate geomagnetic storm) at 01:14 UTC.
G3 – Strong threshold was reached at 01:28 UTC, followed by G4 – Severe at 03:00 UTC.
The G4-level severe geomagnetic storm can have significant impacts, primarily affecting regions poleward of 45 degrees geomagnetic latitude.
Induced currents from such a storm can cause widespread voltage control problems in power grids, and there is a risk that protective systems may mistakenly disconnect key assets. Additionally, intensified induced currents in pipelines may occur.
Spacecraft are likely to experience surface charging, increased atmospheric drag on low Earth orbit satellites, and potential issues with tracking and orientation systems. Navigation services, particularly satellite-based GPS, could be degraded or become inoperable for several hours.
High-frequency (HF) radio communications may become sporadic or suffer complete blackouts. Furthermore, auroral displays might be visible much farther south than usual, reaching as low as Alabama and northern California.
On average, geomagnetic storms of this intensity occur around 100 times per solar cycle (11 years).
In the United States, auroras were sighted as far south as the Texas Panhandle, New Mexico and Florida.
Ahead of the impact, solar winds were already enhanced under positive polarity coronal hole high speed stream (CH HSS), with wind speed averaging around 455 km/s.
The total field ranged from 4 – 10 nT while the Bz component was between +8/-9 nT. Phi angle was predominantly positive.
Following the impact at 22:49 UTC, the total magnetic field increased from 9 – 16 nT, reaching a peak of 18 nT, while Bz turned south reaching -13 nT before turning variable. Solar wind speeds also increased at that time, from approximately 440 km/s to 580 km/s.